Black fly bites won’t give you River Blindness, and mosquitoes can’t transmit Dengue Fever. And yet where (certain species of) these insects reside, the disease is sure to be there. How can this be?
Like the Kissing Bug, these insects act as vectors for pathogenic agents, such as parasites and viruses. They act like planes, shuttling parasites from host to host (airport to airport?). Once in the new host, the parasites go about living a self-indulgent existence, often wreaking havoc on their surroundings and causing all sorts of symptoms in the host (which we lump together and label as a disease).
Mosquitoes, specifically Aedes aegypti, can ingest blood-born parasites and viruses when they bite a person or animal. Most of these will be destroyed by the mosquito’s immune system, but some will make their way into its salivary glands. An example of this sort of parasite is the Dengue virus, which causes Dengue Fever. Likewise, black flies ingest/transmit the parasite Onchocercus volvulus, who causes Onchocerciasis (River Blindness)
EDIT: changed the black fly’s abdomen (made it shorter) and thorax (tried to make it more hump-like).
Also, if you have 5 minutes, I would greatly appreciate it you could complete a survey on science-blog readers! It would greatly help me improve my content, and there are PRIZES. You can read more about it on Survey Rat, or begin the Survey here: http://bit.ly/ScienceBorealisSurvey